The present invention relates generally to oxidation resistant coatings formed of alloys of titanium and aluminum. More particularly, it relates to such coatings formed of alloys of titanium and aluminum which have been modified both with respect to stoichiometric ratio and with respect to chromium and tantalum addition.
It is known that as aluminum is added to titanium metal in greater and greater proportions the crystal form of the resultant titanium aluminum composition changes. Small percentages of aluminum go into solid solution in titanium and the crystal form remains that of alpha titanium. At higher concentrations of aluminum (including about 25 to 35 atomic %) an intermetallic compound Ti.sub.3 Al is formed. The Ti.sub.3 Al has an ordered hexagonal crystal form called alpha-2. At still higher concentrations of aluminum (including the range of 50 to 60 atomic % aluminum) another intermetallic compound TiAl, is formed having an ordered tetragonal crystal form called gamma. Coatings formed of the gamma compound, as modified, is the subject matter of the present invention. Also, the invention concerns titanium aluminide alloy structures which are coated by gamma TiAl modified by chromium and tantalum.
The alloy of titanium and aluminum having a gamma crystal form, and a stoichiometric ratio of approximately one, is an intermetallic compound having a high modulus, a low density, a high thermal conductivity, favorable, although not extraordinary, oxidation resistance, and good creep resistance.
The relationship between the modulus and temperature for TiAl compounds to other alloys of titanium and in relation to nickel base superalloys is shown in FIG. 5. As is evident from the figure, the TiAl has the best modulus of any of the titanium alloys. Not only is the TiAl modulus higher at higher temperature but the rate of decrease of the modulus with temperature increase is lower for TiAl than for the other titanium alloys. Moreover, the TiAl retains a useful modulus at temperatures above those at which the other titanium alloys become useless. Alloys which are based on the TiAl intermetallic compound are attractive lightweight materials for use where high modulus is required at high temperatures and where good, although not extraordinary, environmental protection is also required.
One of the characteristics of TiAl which limits its actual application to such uses is a brittleness which is found to occur at room temperature. Also, the strength of the intermetallic compound at room temperature can use improvement before the TiAl intermetallic compound can be exploited in certain structural component applications. Improvements of the gamma TiAl intermetallic compound to enhance creep resistance as well as to enhance ductility and/or strength at room temperature are very highly desirable in order to permit use of the compositions at the higher temperatures for which they are suitable.
With potential benefits of use at light weight and at high temperatures, what is most desired in the TiAl compositions which are to be used is a combination of strength and ductility at room temperature. A minimum ductility of over one percent is acceptable for some applications of the metal composition but higher ductilities are much more desirable. A minimum strength for a composition to be useful is about 50 ksi or about 350 MPa. However, materials having this level of strength are of marginal utility for certain applications and higher strengths are often preferred for some applications.
The stoichiometric ratio of gamma TiAl compounds can vary over a range without altering the crystal structure. The aluminum content can vary from about 50 to about 60 atom percent. The properties of gamma TiAl compositions are, however, subject to every, significant as a result of relatively small changes of one percent or more in the stoichiometric ratio of the titanium and aluminum ingredients. Also, the properties are similarly significantly affected by the addition of relatively similar small amounts of ternary elements.
I have now discovered that further improvements can be made in the gamma TiAl intermetallic compounds and particularly in oxidation resistance, by incorporating therein a combination of additive elements so that the composition not only contains a ternary additive element but also a quaternary additive element.
Furthermore, I have discovered that the composition including the quaternary additive element has a uniquely desirable combination of properties which include a substantially improved strength, a desirably high ductility, a significantly improved creep resistance, and a remarkably valuable oxidation resistance. In fact, the oxidation resistance is so high that the material can be used as a coating layer on other titanium aluminide alloys of lower oxidation resistance.